High intensity sodium vapor lamps of the present kind are described in U.S. Pat. No. 3,248,590 -- Schmidt, entitled "High Pressure Sodium Vapor Lamp." These lamps utilize a slender tubular envelope of light-transmissive ceramic resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises an amalgam of sodium and mercury along with a rare gas to facilitate starting. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes which may comprise a refractory metal structure activated by electron emissive material. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with usual screw base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the interenvelope space is usually evacuated in order to conserve heat.
In one ceramic lamp construction which has gone into extensive commercial use, each end of the alumina tube is sealed by a refractory metal closure member having a skirt portion which fits around the end of the alumina tube and is bonded thereto by a thin annular layer of glassy sealing material comprising aluminum oxide, calcium oxide, magnesium oxide, and barium oxide. Each end cap supports an electrode extending along the axis of the tube and is an electrical connector to the electrode. At least one of the end caps, that which is located lowermost in operation of the lamp, has a metal tube projecting hermetically through its end which is used as an exhaust tube during manufacture and then pinched off. In an alternative construction taught in U.S. Pat. No. 3,363,134 -- Johnson, a ceramic end cap is used having an externally projecting metal tube which is sealed off at its outer end. During operation of the lamps, the metal tube, which is sometimes referred to as the appendix, has the lowest temperature in the arc tube and becomes the cold spot where unvaporized sodium-mercury-amalgam collects. Its temperature determines the vapor pressure of sodium and mercury throughout the alumina arc tube.
A problem encountered with such lamps, particularly in the smaller sizes less than 200 watts, as in the 125 watt, 100 watt and 70 watt sizes, is too low a temperature at the appendix or cold spot location. In U.S. Pat. No. 3,723,784 -- Sulcs et al., heat reflecting shields in the form of refractory metal foil bands are placed around the ends of the alumina tube next to the metal end caps. These shields are provided primarily to raise the temperature in the region of the seal and prevent amalgam condensation thereat, but they are also effective to raise the temperature of the metal exhaust tube. In general the wider the shield the greater the increase in temperature. However the use of wider shields or foils aone may not raise the temperature of the metal exhaust tube to the desired extent, and also the seal temperature may become excessive before the objective is achieved.